Intraseasonal oscillations and the South China Sea summer monsoon onset

This paper investigates the role of intraseasonal oscillations (ISOs) in the onset of the South China Sea summer monsoon (SCSSM). Two major components of ISO (10-20-day and 30-60-day modes) are identified. The coupling of these two intraseasonal modes during the pre-monsoon period of the SCSSM are investigated by examining the filtered outgoing longwave radiation (OLR), low-level circulation, apparent heat source and apparent moisture sink from October of a previous calendar year to September of a calendar year. The zonal and meridional propagations of the 10-20-day and 30-60-day modes are found to be different, which reflects their different roles in the establishment and development of the SCSSM. The northwestward propagation of the 10-20-day mode is associated with the weakening of the subtropical high over the western Pacific, while the northeastward propagation of the 30-60-day mode originates from convection over the equatorial Indian Ocean. A hypothesis is then proposed to explain the observed variabilities in the SCSSM onset. When the equatorial Indian Ocean exhibits a 30-60-day mode oscillation, an initially weak convection develops into a large convection band (or monsoon trough). Meanwhile, a convective disturbance of the 10-20-day mode is induced when this monsoon trough extends to the western Pacific. These two processes then collaborate to cause a weakening of the subtropical anticyclone over the South China Sea. Because the monsoon trough associated with the 30-60-day mode subsequently propagates northward into the Bay of Bengal (BOB), the induced vortex together with the 10-20-day westward-migrating convection from the equatorial western Pacific will substantially increase the effect of horizontal advection of moisture and heat, thus destabilizing the atmosphere and weakening the subtropical ridge there. Westerlies can then penetrate and prevail over the SCS region, and the SCSSM onset occurs. Copyright (c) 2005 Royal Meteorological Society.